Friedrich Nietzsche, a German philosopher from the 19th century, has long baffled philosophers with his take on the theory of eternal return: the ancient belief that the universe and everything in existence repeats and recurs into infinite space and time. Nietzsche called eternal return “das schwerste Gewicht,” or “the heaviest burden,” and, conversely, he argued that any existence that does not return is “like a shadow, without weight.” Nietzsche’s understanding of the theory assumes that the heaviness of eternal return is negative, while its opposite, lightness, is positive, but the unnamed narrator isn’t so convinced. He questions if heaviness is really undesirable, and points out that “the lightness/weight opposition is the most mysterious, most ambiguous of all” known opposites.
The narrator has been thinking about Tomas and Tereza for years. Tomas is “born” staring out the window of his Prague flat, trying to decide what to do about his feelings for Tereza. Tomas is a perpetual bachelor and libertine, and his life isn’t bogged down with attachments of love and committed relationships. He has managed, however, to fall in love with Tereza, a woman who barges into his life with her “enormously heavy” suitcase and her equally weighty ideas of love and monogamy. Before long, Tomas and Tereza are married, but this doesn’t mean that Tomas has given up his mistresses, especially Sabina, with whom Tomas has been carrying on an affair for years. Tereza knows all about Tomas’s unfaithfulness, and when the couple moves to Zurich after the Prague Spring, Tereza is hoping that Tomas will leave his infidelities behind. He does not, however, and when he continues to see Sabina (who has since immigrated to Geneva), Tereza leaves Zurich and heads back to Prague. Within days, Tomas follows her—even though Czechoslovakia’s Communist state dictates that he won’t be able to leave again. Back in Prague, Tereza feels intensely responsible for Tomas’s decision to return to Prague and change his fate, and she and Tomas both struggle with the implications of his choice.
Meanwhile, it is early afternoon in Geneva, and Franz is on his way to see his own mistress, who also happens to be Sabina. Franz is going to Sabina’s art studio, but he doesn’t plan on sleeping with her there. Franz only has sex with Sabina in foreign countries, as sleeping with both Sabina and his wife, Marie-Claude, in the same country would cheapen Franz’s marriage, as well as his relationship with Sabina. Franz asks Sabina to go with him to Palermo, but she isn’t interested in going anywhere. She would rather stay in Prague, Sabina tells Franz, as she steps out of her skirt and places her black bowler hat on her head. In doing so, she means to tell Franz that she would rather have sex right now, in Geneva, but the significance of the hat is lost on him. Franz’s inability to decipher the meaning of Sabina’s hat is just one of many misunderstandings between Franz and Sabina, and to illustrate this point, the narrator includes a short dictionary of the misunderstood words between them. Franz and Sabina have conflicting definitions for common words such as “woman,” “cemetery,” and “parade,” but their differences are best reflected in their understanding of the word “betrayal,” which Franz considers a “most heinous offense.” Sabina, on the other hand, views betrayals as a “breaking [of] ranks” and an adventure into the unknown. When Franz finally tells his wife about Sabina, Sabina ultimately betrays Franz as well, leaving him alone without wife or mistress.
After leaving Franz, Sabina moves to Paris, but she is plagued by a persistent depression. Sabina’s depression is not the result of “heaviness” or “burden,” the narrator says, “but of lightness,” as she has fallen victim to “the unbearable lightness of being.” Sabina lives in Paris for nearly three years, and then she receives a letter from Tomas’s son, Simon, which informs her of Tomas and Tereza’s deaths in a car accident. At the same time, Franz has left Marie-Claude—although she refuses to agree to a divorce—and is living with one of his young students. Franz, who is in love with parades and the idea of protesting and marching, decides to join a Grand March to Cambodia. The march, consisting primarily of intellectuals, hopes to convince the Cambodian government to let a group of doctors into the country to help the Cambodian people, a country that, like Czechoslovakia, has been devastated by Communism and foreign invasions. When the Grand March reaches the Cambodian border, they are ignored and denied entrance into the country, and Franz’s romantic ideals about the power of protest and resistance are dashed. Before Franz leaves Thailand, on his way back to Europe from the Grand March, he is held up for his money by two men. When Franz resists the robbery, he is struck on the head and later dies at a hospital in Geneva.
Sabina ultimately moves to America, where she lives with an elderly couple in a kind of makeshift family arrangement. While Sabina has spent her entire life trying to avoid heavy commitments and kitsch, she is never able to escape the pull of familial relationships. Sabina makes plans for her ashes to be scattered into the wind upon her death, but it is Tomas and Tereza’s final days the narrator focuses on. After returning to Prague, both Tereza and Tomas are dismissed from their professional jobs and forced to do menial work. They move to the country, far away from the government and Tomas’s mistresses, where they live a comfortable and predictable life with their dog, Karenin. The cyclical nature of their lives in the country—they wake at the same time each day, run the same errands, and go to the same jobs—is the closest Tomas and Tereza come to happiness throughout the novel, but they still aren’t entirely happy. Tereza still obsesses about Tomas’s unfaithfulness, and, what’s worse, she sees him as an old man stripped of his strength and power, like a helpless and scared rabbit in her hands. Sadly, Karenin develops cancer and dies, and while Tereza and Tomas are crushed, they still decide to go dancing in a nearby town. That night, they drink and dance, and Tomas tells Tereza that he doesn’t regret his decision to leave Zurich and return to Prague to be with her. He considers himself “free” and happy, even though their lives have not turned out as expected. They decide to spend the night and return home in the morning, and even though it is clear to the reader that Tomas and Tereza will die in the car accident the next day, there is a general feeling of optimism as they enter their hotel room for the night.